1. Field of the Invention
The present invention relates to a steel cord for reinforcing a rubber product, having good penetration of rubber and high rigidity, and to a pneumatic tire having improved durability, steering stability and uniformity through the use of this steel cord as a reinforcing member.
2. Description of the Related Art
In the field of steel cords for reinforcing rubber products such as tires, steel cords into which an elastic material such as rubber can penetrate have been proposed in order to improve the durability of rubber products reinforced by the steel cords.
For example, as shown in FIG. 5, an open cord 100 has been proposed which is composed of filaments 102 twisted together in large spirals with gaps 104 therebetween in order to permit penetration of rubber into the cord 100.
However, the open cord 100 has drawbacks caused by its low rigidity due to the filaments 102 being twisted together in large spirals. For example, it is difficult to maintain good uniformity of tires when the open cord 100 is used, because the open cord 100 can easily elongate under a low tensile load such as that applied to the cord in the production process of tires.
Further, another drawback of the open cord 100 is that the penetration of rubber is unreliable because the filaments 102 can easily be made to contact each other due to the tension or compression which arises in the production process of rubber products such as tires.
In order to overcome such drawbacks of the open cord, various steel cords have been proposed.
For example, a steel cord, which comprises at least one filament having a spiral shape prior to twisting with other filaments and in which these filaments are twisted together such that the twisting pitch of the steel cord is larger than the spiral pitch of said at least one filaments, is disclosed in Japanese Utility Model Application Laid-Open (JP-U) No. 4-60589. Further, a 1+3 structure in which the core filament 114 has a continuous wave shape (FIG. 6) is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 5-295682.
These steel cords have higher rigidity and exhibit more reliable penetration of rubber than the open cord, and can overcome the drawbacks of the open cord to a certain extent. However, the improvement is not enough to provide excellent steering stability and uniformity for tires, and thus, further improvement are required.